The present invention relates to an absorbent composite capable of handling simple and complex fluids and maintaining high absorbent capacity under high loads even after the material has been subjected to rigorous processing conditions.
Superabsorbent materials possess a number of attributes that make them attractive in many different applications. As a result of their superior water-absorbing attributes, superabsorbent materials have supplanted much of the traditional absorbents in disposable diapers and have made significant improvements in the performance of disposable feminine hygiene products and disposable adult incontinence products. The basic property of water absorption has suggested the use of superabsorbent materials in many other applications, including paper towels, surgical sponges, meat trays, disposable mats for outside doorways and in bathrooms, and for household pet litter, bandages and wound dressings.
The largest use of superabsorbent materials, however, is in disposable personal hygiene products. These products include, in order of volume of superabsorbent material used, diapers, training pants, adult incontinence products and feminine hygiene products. Of these, diapers accounted for over 90% of the total superabsorbent material sold in 1995. Because of this, the development of superabsorbent properties in general has been focused on optimizing absorbency of urine.
A challenge for the developers of products into which superabsorbent materials are incorporated, however, is the very significant difference between the fluids to be absorbed by the various disposable absorbent products. With diapers, for example, the fluid is typically urine, a simple fluid of primarily water, salts and nitrogenous materials such as urea. With feminine hygiene products, for example, the fluid is typically menses, a complex fluid including water, salts, and cells. In such complex fluids, the cells are far too large to diffuse into the network structure of the superabsorbent material, and may instead adsorb onto the surfaces of the particles of superabsorbent material. The high osmotic pressure of partially swollen superabsorbent material can de-water the cells if they are in direct contact and this can lead to a nearly impermeable surface layer of cells surrounding the superabsorbent material, resulting in a severe reduction in the efficacy of the superabsorbent material. These factors suggest that the nature of the superabsorbent material for absorbing complex fluids such as menses should be different from the superabsorbent material used for absorbing simple fluids such as urine. Examples of other complex fluids include blood, runny bowel movements, and nasal discharge.
A further challenge in developing products into which superabsorbent materials are incorporated is the loss of absorbency under high loads caused by rigorous processing conditions. The act of processing materials, such as converting a material into a diaper absorbent core, tends to fracture superabsorbents. Prior to the development of inhomogeneously crosslinked polyacrylate superabsorbents, fracture was actually seen as desirable to soften the absorbent product. In fact, the deliberate breakage of polyacrylate superabsorbents was considered a means of softening the product. However, with the development of inhomogeneously crosslinked polyacrylates (shell-core materials), fracture during processing has become recognized as detrimental to absorbency, as discussed in U.S. Pat. No. 6,214,274.
There is a need or desire for an absorbent material that is capable of handling simple and complex fluids and can withstand rigorous processing conditions without losing absorbency under high loads.
In response to the discussed difficulties and problems encountered in the prior art, a new absorbent composite has been discovered.
The present invention is directed to superabsorbent material with a shell of surface crosslinks. This inhomogeneous crosslinking allows the bulk of the material to have high capacity while the shell prevents deformation under load. The disadvantage of the core/shell structure under rigorous process conditions where the shell is fractured negates the benefit of increased absorbent capacity under loads.
The absorbent composite of the invention includes an inhomogeneously crosslinked polyacrylate superabsorbent material coated with any of a variety of protective materials, such as cellulose fibers, with a variety of association agents to provide a xe2x80x9ccushionxe2x80x9d to protect the superabsorbent during processing. Following rigorous processing conditions, which can be simulated using the Ball-Mill Process described below, the Absorbency-Under-Load (AUL) (0.9 psi) of the superabsorbent is not degraded compared to the AUL of the superabsorbent prior to undergoing the rigorous process conditions. In some cases the 0.9 psi AUL is also enhanced by the coating compared to an uncoated superabsorbent material. This increase is believed to be due to the cushioning of the particle by the fiber on the surface. Fiber may be detached from the surface by ball-milling, so the protection for the coated particles may be slowly removed during processing.
The length of time of the ball-mill procedure can be varied, and 0.9 AULs measured to identify a maximum possible level of protection for possible process variations. The quantity or type of coating can then be modified such that the maximum absorbency occurs under standard converting conditions.
The material of this invention can also handle complex fluids which may be advantageous in certain product executions. More specifically, the coating can adsorb cells, tissue, and proteins, allowing the superabsorbent to effectively absorb the remaining fluid.
The absorbent composite of the invention can be made by combining an inhomogeneously crosslinked superabsorbent polymer, a protective, fibrous coating material, and an association medium. An example of a suitable coating material is cellulose powder. An example of a suitable association medium is water, since slightly wet superabsorbent polymer becomes sticky and, unexpectedly, continues to hold the coating even after drying. Water-soluble additives can also be added to the absorbent composite. With some of these soluble additives, fluid intake rate may increase or odor may be controlled, for example.
The absorbent composite of the invention can be used to make diapers, training pants, and incontinence products that are effective for runny bowel movements, and hyperthin feminine products for menses management, as well as paper towels and tissues that not only absorb more fluid but are also dry to the touch within minutes of absorbing fluid.
With the foregoing in mind, particular embodiments of the invention provide an absorbent composite that is capable of handling complex fluids and can withstand rigorous processing conditions without losing absorbency under high loads.